At present a thermal pyrolysis of hydrocarbons is the basic process of commercial production of low-molecular olefins—ethylene and propylene. As a feedstock there are used hydrocarbons which molecule has two or more atoms of carbon. In industry there are generally used gases of petroleum refining, as well as naphtha and gas oil fractions.
In generally accepted technology a feedstock mixed with steam and preheated, is supplied into a cracking tube located inside radiant section of pyrolysis furnace, wherein the mixture is rapidly heated. Pyrolysis reactions proceed with large absorption of heat. A product stream having the outlet temperature 750–950° C. is quenched and transported into a gas fractionating installation, in which ethylene, propylene, butadiene, methane, hydrogen and other pyrolysis products are separated. Ethylene is the most valuable product of pyrolysis.
Among reactions of thermal pyrolysis there can be distinguished: the primary reactions resulting in formation of olefins and the secondary ones, during which formed olefins are wasted. With temperature increase these reactions are accelerated, both primary and secondary, but the rate of primary reactions increases quicker than the rate of secondary ones. The rate of primary reactions does not depend on pressure, whereas the rate of secondary reactions decreases with pressure decrease. Therefore, to increase the olefin yields there is a need to decrease hydrocarbon partial pressure in the work cavity and increase the process temperature within said limits. The hydrocarbon partial pressure can be decreased by adding a steam-diluent. An optimal amount of steam-diluent depends on the composition of the hydrocarbon feed. For light feedstock—ethane or propane—the amount of steam usually is 20–40% of feedstock mass. For heavy feedstock, such as gas oils, the amount of a steam can be 80–100% of feedstock mass. It is undesirable to increase the pyrolysis temperature above 950–1000° C., because this accelerates sharply coke formation and causes increase in yield of less valuable acetylene yield at the expense of ethylene.
Essential component of pyrolysis plants is the means for quenching of product stream leaving reactor to the temperature of stopping undesirable secondary reactions. The quenching can be both direct—by injection of steam, water or light pyrolysis tar—and indirect—by using a heat exchanger. The direct quenching is usually applied in thermal cracking of gas oils. In thermal cracking of light hydrocarbons the indirect quenching in heat exchanger apparatus is usually applied generating simultaneously a high pressure steam.
During pyrolysis of hydrocarbons a pyrocarbon is always evolved, part of which in the form of soot particles is carried away by the product stream, but the another part forms coke deposits on the walls both of cracking tubes and downstream apparatuses as well. Coke deposits increase a pressure drop through cracking tubes and deteriorate a proper heat transfer into reaction zone, resulting in overheating the cracking tubes, decrease in productivity of pyrolysis plant and decrease in yields of low-molecular olefins. Therefore coke deposits are periodically removed, usually this is performed by burning out with air or air-steam mixture.
Following disadvantages of hydrocarbon pyrolysis in tubular furnaces is noteworthy:
a. it is necessary to transfer a large amount of heat into the reaction zone through the cracking tube walls. Because of large heat flows the temperature of the cracking tube wall much exceeds the temperature of a process stream causing an intensive coke formation and decrease in desired products yield. It is impossible to decrease the pressure in the zone of pyrolysis because of the necessity to provide a high rate of feedstock flow through this zone required by conditions of heat transfer;
b. the rate of feedstock heating through the cracking tube is insufficient. So an initial amount of desired olefins formed at relatively low temperatures and carried further by feedstock flow through more and more intensively heated zones resides an excess time under conditions, when secondary reactions proceed with grate intensity. This disadvantage becomes greatly evident in pyrolysis of wide petroleum fractions, such as naphtha or gas oil, which contain both easy cracked high-molecular hydrocarbons and low-molecular hydrocarbons cracked under higher temperatures.
To eliminate said disadvantages, the processes for producing olefins by the thermal decomposition of hydrocarbons were offered, in which heat is not transferred into the reaction zone through boundary of this zone.
U.S. Pat. No. 5,300,216 discloses method and apparatus for thermal cracking hydrocarbons in the presence of steam by passing through stationary shock wave of high intensity. A steam superheated in a tubular heater to a temperature of about 1000° C. is introduced at a pressure of about 2.7 MPa through a supersonic nozzle into a reactor comprising series-positioned mixing and pyrolysis zones. In the mixing zone the hydrocarbon feed—ethane—preheated to the temperature of about 627° C. is introduced through mixers into supersonic flow of steam. The resulting mixture forms a supersonic process stream, which has a temperature lower than that required to initiate pyrolysis reactions. Between the said mixing and pyrolysis zones a straight compression shock—continuous-standing shock wave—is created. In this compression shock a kinetic energy of the supersonic process stream is converted into the heat. Immediately downstream of the compression shock the velocity of the process stream falls to subsonic level, and the temperature rises up to about 1000° C. at a pressure of about 0.9 MPa abs. The process stream passes the pyrolysis zone for 0.005–0.05 sec. while its temperature decreases to about 863° C. at the expense of heat absorbed by pyrolysis reactions. Conversion of ethane into ethylene achieves 70%. The product stream passes quenching apparatus and downstream heat exchangers, and further is transported to gas fractionation. In this apparatus all said above disadvantages of tubular pyrolysis reactors are eliminated. The feedstock reaches maximum pyrolysis temperature especially rapidly, and the boundary of the pyrolysis section are not used for transfer of heat into reaction zone. But at the same time the required amount of steam per hydrocarbon mass must be about 500–667%. Because of this the energy expenses per unit of produced ethylene are excessively high and can not be essentially decreased. Thus this apparatus noncompetitive at current interrelation of energy costs with olefin costs.
U.S. Pat. No. 5,597,537 discloses a process for fluidized catalytic cracking of hydrocarbon feed stream. This process comprises contacting the hydrocarbon feed stream with a dust catalyst at an initial temperature 664° C. in a riser. Heat required for the cracking is transferred to process stream from particles of catalyst, and temperature of the catalyst decreases to 532° C. Then hydrocarbon products are separated from a coked catalyst, and coked catalyst passes to a regenerator. Here the coke from said catalyst is combusted by hot air to produce regenerated catalyst, and catalyst temperature increases. Regenerated catalyst passes into riser again. In this apparatus a heat is not transferred into reaction zone through boundary of this zone.
Unfortunately, high severity of destruction process, typical for pyrolysis of hydrocarbons in tubular furnaces, in this apparatus is impossible. This is a cause on which the best yield of ethylene in this apparatus does not exceed 5 . . . 7%, and so this apparatus is not used for commercial producing of ethylene.
U.S. Pat. No. 4,265,732 discloses a process for thermal cracking gaseous hydrocarbon feed, wherein the heal required for pyrolysis is generated directly in a process stream due to hydrodynamic drag of the rotated blades. Pyrolysis takes place in a reactor designed as a multistage blade machine of axial type containing alternating rotating blade rows and stationary blade rows positioned in series. Desired profiles of a pressure and temperature along the reactor length are reached by properly profiling of these blades. The reactor boundary is not used for transfer of heat into the reaction zone.
This invention eliminates completely the disadvantage (a), but the disadvantage (b) is not eliminated. An axial type multistage blade apparatus is needed to realize this process. This apparatus have to be capable of work at very high temperatures (up to 1050° C.) and comprise a great number of stages (up to 43). Difficulties appearing in development of this apparatus are so heavy that such apparatus has not been fabricated.
Thus all known methods of low-molecular olefins production by thermal pyrolysis of hydrocarbons when a heat is not transferred into zone of reaction through boundary of this zone are not suitable for commercial using.
Thereby, attempts to develop a method of pyrolysis of hydrocarbons free of disadvantages (a) and (b) were not successful and at present the thermal pyrolysis of hydrocarbons in tubular furnaces is a single commercial method of the ethylene production.
In prior art there are known and widely used hydrodynamic devices, in which a torque is transferred from rotating shaft to stationary housing by ring vortex of medium filling work cavity. In these devices mechanical energy is converted into heat that is generated directly inside work medium and heats it. Such hydrodynamic devices having small overall dimension can transfer a maximum torque and absorb maximum power.
Here and hereinafter wording “ring vortex” means a vortex with the ring core, thereby, word “ring” pertains to the form of vortex core. Streamlines of such vortex are of the form of spirals convolute in ring.
U.S. Pat. No. 2,672,954 discloses hydrodynamic dynamometer. Housing and rotatable rotor located inside it form together two annular work cavities filled with liquid. In the work cavity the blades fixed on the rotor and directing blades fixed on the housing are located. Said cavities are formed and the said blades are configured and positioned so that in each cavity a ring vortex is created when the rotor is rotated. This ring vortex transfers a torque from rotor to housing of device. Cold work liquid is supplied into this device and hot one is withdrawn from the device through respective nipples communicated with work cavity.
U.S. Pat. No. 3,537,264 discloses hydrodynamic brake. Housing and rotatable rotor, which is located inside it, form together an annular work cavity filling with work fluid. This work cavity through respective channels is communicated with inlet and outlet nipples for transporting the work fluid. In the work cavity the rotor blades and stationary directing blades are located. The said cavity is formed and the said blades are configured and positioned so that a ring vortex is created when the rotor is rotated. This ring vortex transfers a torque from rotor to housing of device. The work fluid in this device is a liquid or gas (air, for example).
U.S. Pat. Nos. 4,864,872, 5,147,181, 5,571,975 disclose fluid torque transfer devices, which constructions are similar to the hydrodynamic brake covered by U.S. Pat. No. 3,537,264. The said devices and hydrodynamic brake on U.S. Pat. No. 3,537,264 differ in form and location of work and stationary blades.